Ch. 9-Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue: Structure, Function, and Physiology

Functions of Muscle Tissue

Muscle tissue is essential for various physiological processes in the human body. Its primary and secondary functions include:

• Produce movement: Muscles contract to move the skeleton and body parts.

• Maintain posture and body position: Continuous muscle contractions stabilize the body.

• Stabilize joints: Muscles reinforce and support joint structures.

• Generate heat (thermogenesis): Muscle activity produces heat, helping maintain body temperature.

• Move substances through the body: Muscles propel blood, food, urine, and other substances.

• Support soft tissues: Muscles provide structural support to internal organs.

Types of Muscle Tissue

There are three main types of muscle tissue, each with distinct characteristics and functions:

Muscle Type	Location	Control	Appearance
Skeletal muscle	Attached to bones	Voluntary	Striated
Smooth muscle	Walls of hollow organs	Involuntary	Non-striated
Cardiac muscle	Heart	Involuntary	Striated with intercalated discs

• Skeletal muscle fibers: Long, cylindrical, multinucleated, and striated.

• Cardiac muscle fibers: Branching cells connected by intercalated discs; striated.

• Smooth muscle fibers: Spindle-shaped, single nucleus, non-striated.

Organization of Skeletal Muscle

Skeletal muscle is organized hierarchically from largest to smallest structures:

1. Muscle (entire muscle)

2. Fascicle (bundle of muscle fibers)

3. Muscle fiber (muscle cell)

4. Myofibril (contractile structures within fibers)

Connective tissue coverings:

• Epimysium: Surrounds the entire muscle

• Perimysium: Surrounds each fascicle

• Endomysium: Surrounds individual muscle fibers

Muscle Fiber Structure

Muscle fibers (cells) have specialized structures for contraction:

• Sarcolemma: Muscle cell membrane

• Sarcoplasm: Cytoplasm of the muscle cell

• Myofibrils: Rod-shaped structures containing contractile proteins

• Myofilaments: Two main types—actin (thin filament) and myosin (thick filament)

These proteins interact to produce muscle contraction.

Sarcomere Structure

The sarcomere is the smallest functional unit of muscle contraction, defined as the region between two Z discs (Z lines).

• I band: Light band, contains only thin filaments (actin)

• A band: Dark band, contains thick filaments (myosin) and overlapping thin filaments

• H zone: Central region of the A band with only thick filaments

Neuromuscular Junction (NMJ)

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber, enabling neural control of muscle contraction.

• Axon terminal: End of the motor neuron

• Synaptic cleft: Gap between neuron and muscle fiber

• Motor end plate: Specialized region of the muscle fiber membrane

• Neurotransmitter: Acetylcholine (ACh) is released, binding to receptors on the sarcolemma

Muscle Contraction Overview

Skeletal muscle contraction involves three major steps:

1. Neuromuscular junction signaling

2. Excitation–contraction coupling

3. Cross-bridge cycling

These steps connect nervous system signals to calcium release and muscle shortening.

Sliding Filament Model

During contraction, thin filaments slide past thick filaments, increasing overlap but not shortening the filaments themselves.

• Sarcomere shortens as Z discs move closer together

• Key idea: Filaments retain their length; only their relative positions change

Excitation–Contraction Coupling

This process links the muscle fiber action potential to calcium release from the sarcoplasmic reticulum, initiating contraction.

• Transverse tubules (T-tubules): Conduct action potentials deep into the muscle fiber

• Sarcoplasmic reticulum: Specialized smooth ER that stores and releases calcium ions

• Calcium function: Binds to troponin, exposing myosin-binding sites on actin

Cross Bridge Cycle

The cross-bridge cycle describes the interaction between actin and myosin during contraction:

1. Cross bridge formation: Myosin head binds to actin

2. Power stroke: Myosin head pivots, pulling actin filament

3. Cross bridge detachment: ATP binds to myosin, causing detachment from actin

4. Myosin head reactivation (cocking): ATP hydrolysis repositions the myosin head

• ATP is required for: Detachment, reactivation, and powering contraction

Energy for Muscle Contraction

Muscle contraction requires significant ATP, supplied by several metabolic pathways:

• Creatine phosphate system: Rapid ATP regeneration for short bursts

• Anaerobic glycolysis: Produces ATP without oxygen, yielding lactic acid

• Aerobic respiration: Most efficient ATP production, requires oxygen

Aerobic respiration produces the most ATP for sustained activity.

Types of Muscle Contractions

• Isometric contraction: Muscle tension increases, but muscle length remains unchanged

• Isotonic contractions:

  • Concentric: Muscle shortens as it contracts

  • Eccentric: Muscle lengthens while contracting

Muscle Tone

Muscle tone is a sustained, partial contraction of skeletal muscles, essential for:

• Maintaining posture

• Stabilizing joints

• Keeping muscles ready for action

Fascicle Arrangements

Fascicle arrangement affects muscle shape and function:

• Parallel: Fibers run parallel to the muscle’s long axis

• Convergent: Fibers converge toward a single tendon

• Circular: Fibers form rings around openings (e.g., orbicularis oris)

• Pennate:

  • Unipennate: Fibers on one side of tendon

  • Bipennate: Fibers on both sides of tendon

  • Multipennate: Multiple featherlike fascicle groups

Muscle Roles in Movement

• Prime mover (agonist): Main muscle responsible for movement

• Antagonist: Opposes or reverses the movement

• Synergist: Assists the prime mover

• Fixator: Stabilizes the origin of the prime mover

Levers in the Body

The musculoskeletal system acts as a system of levers:

• Bones: Act as levers

• Joints: Serve as fulcrums

• Muscles: Provide the effort (force)

• Most muscles operate as third-class levers, where the effort is applied between the fulcrum and the load.

Example: The biceps brachii flexing the forearm at the elbow is a classic third-class lever system.